In computer-based image composition and editing systems, which store imagery data in digital format, it is common practice to selectively combine plural images by merging or inserting a first image, termed the `paste` image, into or on top of a second image termed the `base` image. Because the images are different, along the edge or border between the base image and the pasted image, an undesirable boundary artifact is created. This anomaly is particularly pronounced where the border has an arbitrary shape, due to the quantized format of the image data base and the finite matrix configuration of the display pixels. One possible mechanism for reducing the artifact is to locally blur the edge of the paste image, so as to `soften` the effect of the image discontinuity. It has been found, however, that such a solution simply produces the obvious--a pasted image with a blurred border.